a, Two modules separated by ∼1 m each contain an ion trap. High numerical aperture objectives couple spontaneously emitted photons from a single atom into a single-mode optical fibre. The photons from atoms in separate traps interfere on a 50/50 beamsplitter (BS), are sorted by polarizing beamsplitters (PBS), then detected by photomultiplier tubes (PMTs). Coincident detection of photons on specific PMT pairs heralds entanglement of atomic spins (Methods). b, Schematic of a large-scale, modular quantum network of trapped ions. Ion trap modules (red boxes) confine atoms coupled together through their Coulomb bus, and entanglement within modules is accomplished with the application of spin-dependent forces on the trapped atoms4. Probabilistic, heralded entanglement is generated between modules via interference of emitted photons from each module. A reconfigurable N × N cross-connect switch links arbitrary modules. Photon interference occurs at fibre beamsplitters, and a single-photon detector array heralds entanglement of atomic spins between modules.